A scroll compressor comprises two disk-like end plates, each having a spiral wrap at one side thereof, facing each other. The two wraps are in contact along several contact lines, forming a plurality of compressor chambers therebetween. In the scroll compressor, one end plate revolves around the other stationary end plate in an eccentric orbit, so that the contact lines gradually shift from the outer circumference toward the inner circumference. The gas that is drawn into the compression chambers between the two wraps is gradually compressed from the outer circumference toward the inner circumference.
There are basically two types of scroll compressor: a lower pressure type, in which the inside of the vessel is maintained at lower pressure, as in U.S. Pat. Nos. 3,011,694 and 4,065,279, and a higher pressure type, in which there is a higher pressure chamber on the opposite side to the compression chamber of the orbiting end plate, as in U.S. Pat. Nos. 3,884,599 and 3,994,633.
In general, in a higher pressure type scroll compressor, a rotation drive device such as a motor and a compression device to compress the gas are installed inside a sealed vessel. The gas (such as air) to be compressed passes through a guide tube which is inserted into the sealed vessel, and enters the compression chamber from one or more inlets on the outer circumference of the compressor. After the compressed gas at a high pressure from the compression chamber has passed through each part of the interior of the sealed vessel, it is exhausted out of the sealed vessel to the outside. That is to say, high-pressure gas which has left the compression chambers between the pair of stationary and orbiting end plates passes around to a first surface, that is, the surface opposite the compression chamber, of the orbiting end plate and a strong force then act on the other stationary end plate.
Consequently, the friction force between the two end plates becomes large, generating heat, and an increase of the drive input becomes necessary. For this reason, heat is again generated by friction, causing the problem that the intake gas is heated before it is drawn in the compression chambers from the intake ports. Also, in a higher pressure type scroll compressor, since the inside of the sealed vessel is at high pressure, the gas density becomes large, causing the problem that large resistance is produced when the Oldham's ring reciprocates between the orbiting end plate and the frame for supporting the end plates inside the sealed vessel.
The lower pressure type is used in small compressors and the end plates used in them are thin, but in the higher pressure type the end plates are thick and inflexible so that they cause a problem with the sealing during operation. A number of methods have been tried to deal with this problem. However, it has never been suggested to use the higher-pressure type in a small compressor and to build a lower-pressure chamber into the higher-pressure chamber.